Polypropylene is widely used because of its high versatility and relatively good temperature resistance. Its rigidity and low density makes it a cost-effective material of choice for many injection molded articles across a range of applications. Today the lack of toughness in polypropylene homopolymers, especially at low temperatures, is overcome by adding in-reactor rubbers. The resulting impact copolymers do have a very good stiffness/impact balance and are available at melt flow rates (230° C., 2.16 kg) from fractional MFR up to more than 100 g/10 min. An unmet need in injection molding applications requiring both stiffness and toughness is a polypropylene that provides good optical properties, such as low haze and high clarity, and shows less sensitivity to stress whitening. The combination of properties should be available at a high melt flow rate to allow for short cycle times and advantaged economics in the production of thin-wall (500 μm to 2.0 mm) injection molded articles.
Historically transparent articles are injection molded with clarified random polypropylene copolymers with reduced crystallinity. The lower stiffness compared to homopolymers can be compensated by the article design. As in polypropylene homopolymers the lack of toughness, especially at low temperatures, requires the addition of at least one rubber component for most applications; especially, where the articles will be transported at low temperature.
Compositions containing blends of random polypropylene copolymers with low crystallinity polyethylenes, e.g. metallocene polyethylene, require relatively high levels of impact modifier in order to achieve a significant improvement in impact resistance. The addition of these high levels of impact modifier is difficult in an injection molding machine and results in an unacceptable cost increase.
Commercial products are available which comprise in-reactor ethylene-propylene rubber and a random polypropylene copolymer. These products are less transparent compared to unmodified random copolymers, offer a fixed level of impact resistance and are limited in melt flow rate. In thin-wall injection molding applications the use of such products leads to higher raw material cost and higher conversion cost due to longer cycle time when compared to random polypropylene copolymers.
What is desired is a composition that can be readily injection molded into articles that exhibit a superior balance of stiffness, excellent optics (such as low haze and/or high clarity), and acceptable toughness levels, while also providing for lower raw material cost and lower conversion cost due to competitive cycle times. Additionally, it is desirable for the articles to resist stress whitening when manipulated and handled.
Objects of the Invention
One object of the invention is to provide a composition which can be made into injection molded articles exhibiting an excellent balance of stiffness, toughness and optical properties, such as high clarity and/or low haze.
Another object of the invention is for the injection molded articles to exhibit the above-mentioned properties and, further, exhibit good resistance to stress whitening.
A further object of the invention is to provide a composition containing an impact modifier that is compatible with clarified polypropylenes and therefore can be dry blended in an injection molding machine, and eliminates the need for long mixing times.